This invention relates to a drop-out compensation system for the replay of television signals recorded on magnetic tape and more particularly to the reproduction of colour picture information recorded on magnetic tape. The term "drop-out" is a known term in the television signal production art referring to the loss of signal occurring as a result of discontinuities in a signal caused by inconsistencies in the recording medium. Such inconsistencies may occur as a result of dirt on the magnetic tape or possibly as a result of a gap in the oxide layer of the tape.
The effect of drop-outs on a displayed television picture is to cause flashes in the displayed picture. To minimise the visibility of flashes on the displayed picture, various techniques are used to compensate for the effects of drop-outs. The missing picture signal can be replaced by:
A. a fixed luminance level preset to some value between black and white, PA1 B. a luminance value equal to the mean level of the picture over some period prior to the drop-out, PA1 C. repetition of the picture signal line preceding the drop-out by the use of some storage medium.
The best form of compensation is provided by method (c) in which a delay circuit or store of one picture scanning line duration is continuously fed with the replayed video signal; when a drop-out occurs the signal at the output of the one line delay circuit is substituted, so that the missing picture information in a scanning line is replaced by that from the scanning line above that in which the drop-out occurred.
In particular, when handling colour television signals recorded on magnetic tape it is desirable that the drop-out compensation should re-insert a signal of the correct hue and saturation. However, the colour information, as is well known, is carried as the modulation of a subcarrier whose frequency bears a non-integral relationship with the line scanning frequency. In fact, in the case of the American NTSC colour system, the subcarrier frequency is a multiple of the line scanning frequency plus a half scanning line frequency cycle. Therefore, since in any one scanning line there will be 227.5 cycles of the subcarrier frequency the same colour information on adjacent lines has a 180.degree. phase relationship. It follows, therefore, that a simple drop-out compensator of the type used in the method (c) above when reinserting the colour picture information from the previous line, would insert the correct luminance and colour saturation values but with the complementary hue. One solution to this problem is known to lie in the shortening of the one line delay time by the period of one half cycle of subcarrier frequency, so that the reinserted colour information has the correct phase. Unfortunately, the reinserted picture information is offset horizontally by the same amount, which may for example result in a niche in a vertical line in the displayed picture.
When processing such television signals in digital form, it is customary to sample and quantize the analogue signal into the digital form at a frequency three times the colour subcarrier frequency. This implies that the combined luminance and modulated chrominance information is described by a digital word three times during each cycle of the subcarrier. Since the digital sampling process occurs regularly, it follows that the chrominance information is described at intervals of 120.degree. of the subcarrier. When using the quantized NTSC colour signal from the previous scanning line to compensate for drop-outs, the signal must be suitably processed to obtain the correct relationship between the reinserted signal and the picture information of the presently scanned line. A horizontal shift of the combined luminance and chrominance information equivalent to 180.degree. of the subcarrier is not possible since the signal is described at 120.degree. intervals. Furthermore, a time displacement of the reinserted signal of 120.degree. or one clock period would result in the reinserted colour information being demodulated and displayed with a 60.degree. phase error.